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 A144291 Triangular numbers n*(n-1)/2 with n and n -1 nonprime. 5
 0, 36, 45, 105, 120, 210, 231, 300, 325, 351, 378, 528, 561, 595, 630, 741, 780, 990, 1035, 1176, 1225, 1275, 1326, 1485, 1540, 1596, 1653, 1953, 2016, 2080, 2145, 2346, 2415, 2775, 2850, 2926, 3003, 3240, 3321, 3570, 3655, 3741, 3828, 4095, 4186, 4278 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 LINKS Robert Israel, Table of n, a(n) for n = 1..10000 FORMULA a(n) = A000217(A068780(n-1)), n>1. - R. J. Mathar, Dec 10 2008 EXAMPLE If n=1, then 1*(1-1)/2=0=a(1). If n=9, then 9*(9-1)/2=36=a(2). etc. MAPLE p:= -1: Res:= NULL: count:= 0: while count < 100 do q:= p; p:= nextprime(p); if p - q > 2 then count:= count + p-q-2; Res:= Res, seq(k*(k+1)/2, k=q+1..p-2); fi od: Res; # Robert Israel, Jul 03 2018 MATHEMATICA Reap[For[n = 1, n <= 100, n++, If[!PrimeQ[n] && !PrimeQ[n-1], Sow[n(n-1)/2] ] ] ][[2, 1]] (* Jean-François Alcover, Mar 27 2019 *) CROSSREFS Cf. A000040, A000217, A068780, A141468. Sequence in context: A168107 A048034 A195528 * A068143 A264961 A188630 Adjacent sequences: A144288 A144289 A144290 * A144292 A144293 A144294 KEYWORD nonn AUTHOR Juri-Stepan Gerasimov, Dec 01 2008 EXTENSIONS 3570 inserted by R. J. Mathar, Dec 10 2008 STATUS approved

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Last modified May 30 10:51 EDT 2023. Contains 363050 sequences. (Running on oeis4.)